1. Field of the Invention:
This invention relates to multistage hydraulic machines and more particularly to multistage hydraulic machines so constructed that the main shaft is capable of being divided into an upper main shaft and lower main shaft at the location of the lowest pressure stage runner, and the lowest pressure stage runner and lower main shaft can be installed or removed through a path at the side of the pit.
2. Background of the Related Art:
In general, with increases in the capacity and head of pumping-up power stations, the limitations, both from the point of view of efficiency and from the point of view of manufacture, of single-stage hydraulic machines are exceeded and it becomes necessary to employ multi-stage hydraulic machines. The main shaft of such multistage hydraulic machines tends to be longer than that of a single-stage hydraulic machine since the multistage machine has a plurality of runners mounted with a separation in the axial direction on the main shaft. This increases the external hydraulic force that acts on this plurality of runners during operation. The result is that a guide bearing at a single location near the runner, as is done in a single-stage hydraulic machine, no longer provides sufficient support strength. Previous practice has therefore been to support the main shaft by the provision of a bottom guide bearing around the main shaft at the point where it passes through the draft tube from the lowest pressure stage runner.
When the main shaft of the hydraulic machine reaches such a length, centering and machining of the main shaft gets difficult, and there are various restrictions on transporting the shaft from the factory to the installation site after machining. Also, when the main shaft of a hydraulic machine gets long, the following problems arise regarding its installation in the power station and its maintenance. Specifically, when assembling the hydraulic machine at the power station or when dismantling it for maintenance and inspection, the main shaft must be raised together with the runners of each stage using a crane in the power station building. This means that the crane has to be of large capacity and the height of the building has to be increased, as a natural consequence of the need to provide a large lifting height below the crane for the long main shaft of the hydraulic machine. The building therefore has to be of very strong construction, increasing its construction cost.
Furthermore, in a multistage hydraulic machine, the lowest pressure stage runner and the portion of the main shaft that extends below this lowest pressure stage runner are more liable than the high pressure stage runners to be damaged due to cavitation and abrasive damage due to earth and sand.
The repair or replacement frequency of the lowest pressure stage runner and the part of the shaft extending below it therefore tends to be higher than that for other parts.
Thus, when a one piece main shaft is used, every time it is dismantled, all the structural items such as the top cover, guide vane opening and closing mechanism, or return vanes must also be dismantled. Also, on reassembly, the same installation work is required as when the power station was initially commissioned, involving extensive centering and locating operations, and so requiring enormous amounts of time, labor and expense for such dismantling and assembly.
Also, when the portion of the integrally constructed main shaft that extends below the lowest pressure stage runner is damaged, occasionaly the whole main shaft must be replaced by a new one, thus requiring enomous amounts of expense for such replacement.